The invention relates to a ceramic filter comprising at least two stripline resonators in the form of printed metal layers which, during operation of the filter, are electromagnetically coupled and which are separated by means of a ceramic dielectric. The invention also relates to a method of manufacturing such a ceramic filter.
Ceramic filters are used, in particular, in transmitters and receivers for high-frequency signals, such as receivers for GSM, PCN and DECT systems. These systems utilize high-frequency signals in the MHz range. For example, GSM (Global System for Mobile Communication) operates in the 900 MHz band, PCN (Personal Communication Network) utilizes a frequency of 1800 MHz and DECT (Digital European Cordless System) also utilizes a frequency of approximately 1800 MHz. Said filters are used, in particular, to suppress undesired signals whose frequency is outside the frequency range used. This is necessary to preclude overloading of the receiver by strong transmitters outside this frequency range.
A ceramic filter of the type mentioned in the opening paragraph as well as a method of manufacturing said filter are known per se. A description thereof is given, for example, in European Patent Application EP-A 541.397. Said Patent Application describes, more particularly, a laminated filter which comprises two stripline resonators of silver or copper and which are provided on a ceramic foil by means of a printing technique, i.e. screen printing. During operation of the known filter, electromagnetic coupling takes place in the plane of the resonators (x,y coupling).
In practice, it has been found that the insertion losses of the known filter are relatively high. This leads to an undesirable reduction in sensitivity of receivers in which said known filter is used.